As the demand for lithography-based device structures having ever-smaller features continues to increase, the need for improved illumination sources used for inspection of the associated reticles that lithographically print these ever-shrinking devices continues to grow. One such illumination source includes an extreme ultraviolet (EUV) light source. One method of creating EUV light includes spinning a cylinder coated with a uniformly thick layer of solid (frozen) plasma-forming target material, such as xenon, and exposing the xenon-coated portion of the cylinder with a pulsed laser suitable for exciting the xenon to generate plasma. In addition, prior to the next pulse of illumination, the cylinder must be rotated and/or translated to expose a fresh region of solid xenon. As the cylinder rotates, gaseous xenon may be sprayed onto the cold non-illuminated portion of the surface of the cylinder, reforming the frozen xenon layer at previously-illuminated spots in order to fill the portions of the xenon ice consumed by the laser over time. A particular spot or zone is not exposed again until sufficient time has elapsed, allowing for the solid xenon surface to return to its original condition. Adjacent spots must be separated by some minimum distance in order to prevent damage to the cylinder surface.
The plasma creation takes place in a fixed location because the associated collection optics must be arranged around a known location and cannot “slew” around to follow a moving plasma source, which would cause mirror distortion. The need for the plasma to appear in a fixed location prevents the use of a moving illuminator laser spot. This creates a challenge in applications involving EUV light sources. To allow maximum operational time and inspection uniformity, a reticle inspection tool should have a source of pulsed EUV light that is not interrupted, but rather runs at a chosen pulse frequency for a long period of time (e.g., hours), which is required to inspect a reticle.
The creation of EUV light using solid xenon on a rotating cylinder has generally been accomplished with two methods. First, the cylinder, rotating at a constant speed, moves slowly in the axial direction from one end to the other, creating a helix of spots along the cylinder. When the helix intersects the top of the usable length of the xenon ice, the illuminating laser is blocked or turned off until the cylinder is moved back to the other end of travel. The total length of time for the helix of exposure plus the retrace time must be sufficient to allow the xenon ice to reform. In the second method, the pulse rate of the illumination laser is held low enough and/or the cylinder is of a large enough diameter that the spots can overlap after one revolution (e.g., forming a single ring around the cylinder at one axial location).
These methods have several deficiencies, especially as applied to generation of uninterrupted EUV illumination. The first method requires the illumination source to be blocked periodically (on the order of every few seconds). This may force, for example, a reticle inspection machine to stop inspecting and then restart after the cylinder is reset, impacting stability and creating overlay issues. The second method requires either a very low pulse rate, which may cause unacceptably long inspections of a reticle, or a very large diameter cylinder, which may cause mechanical vibration instability issues, or both. Therefore, it is desirable to provide a method and system that cure the defects of the prior art identified above.